Electro-magnetic lifting magnets are commonly associated with cranes. Cranes with lifting magnets are utilized for manipulating relatively heavy magnetic materials, such as, for example, scrap steel, ferrous material, and the like.
In operation, if electric current is delivered, without interruption, to the lifting magnet, the lifting magnet generates heat which detracts from its magnetic strength. To compensate for this loss of magnetic strength, the operator often increases current flow to the magnet. The increased current flow may solve the immediate problem by re-establishing the magnet's strength; however, it exacerbates the heating of the magnet due to I2R losses generated in the windings of the lifting magnet. If this current escalation is carried out to an extreme, it can lead to destruction/failure of the lifting magnet.
An experienced crane operator may, however, manipulate the electromagnet controls in other ways in an effort to manually establish an efficient operation of a crane/lifting magnet combination. For example, an efficient operation of a crane can be manually controlled by the operator by manipulating the timing of an energize-to-de-energized duty cycle period (i.e., a rest period) of a lifting magnet during each load-unload-reload cycle (hereinafter lift cycle). The “load” portion of the lift cycle may be, for example, thirty seconds long and the “unload” period (i.e. the period between unloading and reloading) may be, for example, three seconds long. As such, an operator may be able to regain a certain efficiency by manually reducing the current to the magnet during the unload period. Of course, the relationship between duty cycle and loss of efficiency is generally not linear.
If a crane operator falls behind schedule, the crane operator may not appropriately time or otherwise provide the lifting magnet with a rest period, thereby causing the lifting magnet to overheat due to a constant, high current that passes through the lifting magnet when it is energized. If the electro-magnet is utilized for a long period of time during a daily shift (without appropriately apportioning the rest period in each lift cycle), an over-heating condition may result in a temporary failure of the lifting magnet. Even further, if this operation is practiced in a similar manner over a protracted period, the repetitive over-heating condition may result in permanent damage to the lifting magnet.
In addition, several drawbacks including, for example, voltage spiking of a hoist motor and whipping of the crane derrick may occur should a crane operator improperly de-energize a lifting magnet during a condition when a crane's hoist motor is generating high torque during a lifting operation.
Accordingly, there is a need in the art for a method and apparatus for improving the control of a crane magnet.